The present invention generally relates to an electron paramagnetic resonance spectroscopic method, particularly to the method for quantifying molecular oxygen and its isotopes in a sample and electron paramagnetic resonance spectrometric cell.
Sophisticated techniques such as mass spectroscopy, infrared spectroscopy, ultraviolet spectroscopy and nuclear magnetic resonance spectroscopy are routinely used by chemists for various analytical applications. Electron paramagnetic resonance spectroscopy, or EPR, is a method of spectroscopic analysis that employs the resonance absorption of microwave energy by paramagnetic molecules or ions in a static magnetic field. EPR is also oftentimes referred to as electron spin resonance spectroscopy or ESR.
EPR (ESR) has become a powerful tool in the study of a wide range of organic compounds, reactions and biochemical processes. EPR also has been applied for investigation of reactions resulting in release or consumption of molecular oxygen (Swartz and Glockner 1989; Sarna et al., 1980; Subczynski and Hyde, 1981; Ligeza et al., 1994; Liu et al, 1995). In these methods, O2 dissolved in liquids is measured indirectly by detection of an EPR signal from spin probes whose signal intensities have been correlated with the concentration of dissolved oxygen. Accordingly, these methods provide no data regarding isotope composition of molecular oxygen.
The X-band EPR signal of molecular oxygen in a gas phase was first observed in 1949 by Berlinger and Castle (Berlinger and Castle, 1949). At room temperature more then 99.9% of gas-phase O2 molecules are in vibrationally nonexited ("ugr"=0) electronic ground 3xcexa3gxe2x88x92 state (Herzberg, 1950), revealing at pressure below 0.5 torr a multiline EPR signal shown in FIG. 1. The multiline EPR signal results from Zeeman transitions (K, J, M)xe2x86x92(Kxe2x80x2, Jxe2x80x2, Mxe2x80x2) of the total angular momentum J, which is formed by coupling between the electronic spin, S=1, and rotational angular K momentum of the O2 molecule, J=S+K (Kramers, 1929; Van Vleck, 1929; Hebb, 1936; Schlapp, 1937; Meckler, 1953; Mizushima and Hill, 1954; Evenson and Mizushima, 1972; Steinbach, 1973; Steinbach, 1975).
An EPR method for determination of the isotopic composition of O2 in a gas phase has been described by Bjerre and Larsen (Bjerre and Larsen, 1983). In this method samples of 18O-enriched KBrO3 were placed in a quartz tube inserted in the microwave cavity of an EPR spectrometer and molecular oxygen was produced by thermal decomposition of KBrO3. Accuracy of the isotopic composition of gaseous oxygen (16O2, 16,18O2 and 18O2) was determined by measurement of relative intensities of selected O2 EPR lines in the range of 1-2%. The method, however, does not quantify the concentration of the oxygen isotopes in the sample and is limited to analysis of gaseous samples.
Accordingly, there is a need for method, which directly quantify O2 isotopes, not only in gaseous samples, but also in liquid or solid samples.
An object of the present invention is to provide a sensitive method for determination of concentrations of different isotope forms of molecular oxygen by electron paramagnetic resonance without destruction of a sample. The method comprises a freezing of a liquid or solid sample placed in a sample finger of an oximetry cell; pumping out the atmosphere within the cell to a pressure below 0.01 torr; warming the frozen sample, so that any dissolved oxygen in the liquid or solid sample is released under vacuum in a gaseous phase into EPR finger portion; refreezing the remaining sample in the sample finger by placing the sample finger in the coolant mixture; placing the EPR finger wherein any oxygen gas resides in an X-band electron paramagnetic resonance spectrometer; and measuring the electron paramagnetic resonance spectrum of the oxygen molecules in the gas phase.
Another object of the present invention is to provide an oximetry cell comprising a closed sample finger connected by capillary to an EPR finger that is placed in the cavity of the EPR spectrometer.
With the foregoing and other objects, advantages and features of the invention that will become hereinafter apparent, the nature of the invention may be more clearly understood by reference to the following detailed description of the invention, the appended claims and to the several views illustrated in the drawings.